Spas, sometimes known as hydrotherapy pools, have enjoyed a wide popularity and are utilized both for therapy and recreation/relaxation. Such spas typically contain as much as 500 gallons of water, which must be heated from ambient to approximately 100 to 105 degrees farenheit. The necessary heat is typically provided by a separate heater, comparable to a swimming pool heater, and fired by gas or an electric power, depending upon the utilities available at the spa location. Such heaters normally take the full flow of the spa circulating pump and heat the water passing through the heater by a few degrees per pass. Because they pass a high volume of water, they must be located immediately adjacent to the spa to avoid high pumping losses. Thus, a separate gas line or high wattage electric line must be run directly to the spa area. The expense of this separate heating installation is a significant proportion of the total cost of the spa. Because of the much higher energy cost efficiency, gas heaters are preferred where gas is available. The use of a gas heater requires a base typically of concrete, and then installation involving placement of the necessary gas line and plumbing connections to and from the spa. Lastly, provision must be made for the dispersal of the flue gasses from the spa away from combustible surfaces. The high cost of these installations discourages their use even with permanent installations where long piping runs must be made. Gas heaters are particularly difficult to cost justify as a retrofit for an existing spa. Such installations are even more impractical in association with portable spas, and as a result portable spas rely almost exclusively on electric heat.
As a result of the high cost of a separate heating installation, it would be desirable to utilize an existing source of heat that is available in the typical home for heating spas. The use of an existing heat source typically requires a heat exchanger, and a liquid-to-liquid heat exchanger is necessary to obtain a practical cost/efficiency ratio. Thus the only source of heat in a typical home potentially suitable for heating the spa is the conventional hot water heater. Conventional hot water heaters have not been adapted for this purpose perhaps because the heater is typically located at a site remote from that utilized for the spa, which would result in a high pumping losses and high thermal losses in bringing the hot water to a heat exchanger located within the spa. Accordingly, the heat exchangers which are available for installation in the spa have not generally seen application to the conventional hot water heater, but rather have been utilized exclusively in association with solar systems. The typical remote relationship between the conventional hot water heater and spa also results in long electrical runs to any control device on the hot water heater. Further, it has been assumed that the conventional hot water heater cannot heat the typical spa in a reasonable period of time or while maintaining sufficient water temperature for domestic uses.
Therefore it is desirable to have a spa-domestic hot water heat exchanger that efficiently transfers heat to the spa water without excessive pumping or thermal losses and which does not require control connections between the spa and hot water heater locations.